With the introduction of glutaraldehyde preservation of biological tissue, and in particular porcine bioprosthetic heart valves, it has become possible to: (a) overcome the poor performance of early formaldehyde-preserved implanted tissue valves; (b) discontinue the use of homograft valves; and (c) avoid the undesirable use of anticoagulants required to prevent thromboembolism associated with the use of non-bioprosthetic (mechanical) heart valves, especially in children. Not unlike similarly important discoveries, however, it appears that the glutaraldehyde-preserved bioprosthesis has created its own dilemma.
Although the relatively biologically inert glutaraldehyde-preserved valves of Carpentier and others have demonstrated excellent long-term durability in most instances, serious drawbacks such as tissue-fatigue and a propensity toward calcification have occured. Moreover, it was initially contemplated that children and adolescents would be among those deriving the greatest benefit from the glutaraldehyde-preserved bioprosthetic heart valves since the anticoagulants required with mechanical prosthesis could be eliminated. Results from an increasing number of recent clinical studies indicate that severe calcification of these tissues with relatively short-term failure is prevalent among children and adolescents. Thus, despite their long-term durability and overall reduced incidence of complications, these glutaraldehyde-preserved valves have been deemed by some to be unsuitable for use in children.
The causes of the calcification of these bioprostheses are not fully understood. It previously has been shown that a variety of factors, including calcium metabolism diseases, age and diet of the patient, and degeneration of tissue components such as collagen all are involved to a certain extent. Recently, the occurence of a specific calcium-binding amino acid (gamma carboxyglutamic acid), laid down after implantation of glutaraldehyde-preserved porcine xenographs, has been demonstrated, and it has been postulated to play a role in calcification. While calcification has been accompanied by degradative changes in the glutaraldehyde-treated collagen fibers of the implanted tissue, it remains unclear whether the dystrophic calcification is a cause or the result of tissue degeneration. Nevertheless, there has been a continued effort to determine the source of the calcification problem with implanted tissue.
Accordingly, it is an object of the present invention to provide a method for effectively reducing the calcification of implanted biological tissue.